Thermoplastic copolymers, particularly polymethyl methacrylate, polystyrene, and methyl methacrylate-styrene copolymers, have desirable characteristics such as excellent transparency, mechanical properties and processability and hence have widely been used in various fields such as automobile parts, electrical parts, industrial parts, miscellaneous goods, and the like.
However, with the expansion of the utilities of these materials, it is required to obtain resin materials having more improved heat distortion resistance and more improved transparency and mechanical properties. Extensive investigation has been conducted for obtaining materials having such required properties. For instance, it is known that a polymer having excellent heat distortion resistance can be produced by copolymerizing methacrylic acid and methyl methacrylate and/or styrene. Generally speaking, however, a methacrylic copolymer resin has high water absorption properties and hence shows low heat resistance due to the water absorption, and further, when the resin is processed to obtain formed products, the products have unsatisfactory splash (stripe, etc.) on the back surface thereof due to generation of volatile materials induced by dehydration during the forming and processing steps.
In order to eliminate such a drawback, it is proposed in U.K. Pat. No. 1,437,176 that a certain amount of an ethylenically unsaturated carboxylic acid group copolymerized in the polymer is converted into a carboxylic anhydride group, and thereby, there can be produced a heat resistant thermoplastic copolymer which can give a formed product having no splash on the back. It is also known that such a polymer is advantageous in high resistance to crazing induced by solvents and further resistance to water. However, in order to convert sufficiently the carboxyl group into anhydride group, it is required to pass the copolymer through an extruder repeatedly, and hence, this process has less productivity and is hardly practiced on an industrial scale.
Japanese Patent first publication (Kokai) No. 217501/1983 discloses a novel copolymer comprising mainly methacrylic acid units which has excellent heat resistance and heat stability. According to the working examples in this literature, the copolymer is produced by copolymerizing methyl methacrylate and/or styrene and methacrylic acid by a continuous bulk polymerization and transferring the resulting copolymer and unreacted materials to a vacuum chamber at a high temperature, wherein the unreacted materials are removed and a six-membered cyclic anhydride group is formed. However, it is necessary to retain the copolymer resin within the high temperature vacuum chamber for a long period of time in order to form the six-membered cyclic anhydride group, which results unfavorably in coloring of the polymer.
Japanese Patent first publication (Kokai) No. 87449/1977 discloses a process for the production of a methacrylic resin composition having excellent fluidity and heat resistance by using an unsaturated carboxylic acid. However, according to this process, a smaller amount of unsaturated carboxylic acid is copolymerized, and hence, the improvement of the heat resistance is insufficient.
Thus, the known copolymers have various problems such as high water absorption properties, bad appearance of the formed product, low productivity, coloring of the product, less improvement in heat resistance, and the like.